ci: upload client with bundled server as artifact

[samcowger]

Following up on #130, which has gone stale. This aims to copy the server binary and Cerberus runtime files directly into cn-client, bundle the client with those files included, then upload the resulting .vsix file. I hope this will allow a user to download that one file and install it directly, without having to build the client or the server themselves. Because these files are included with the client in a known location, the client will know how to point the server at them, which solves the problem mentioned in #130 (comment).

This PR is a draft for a few reasons:

• It involves CI experimentation, so I anticipate pushing changes to it to see how CI reacts
• This only compiles the server and client in Ubuntu, but if that works, I'll want to update it to do so in macOS
• This doesn't bother trying to statically link the server binary (e.g. by following the procedure in "Generating static and portable executables with OCaml"), and I don't know how/if that binary will break if it's downloaded and run locally